Carburetors are widely used to produce and control the mixture of fuel and air delivered to an operating engine. Current carburetors utilize one or more needle valve assemblies to meter the quantity of fuel in the fuel and air mixture. The needle valve assemblies have a pin or needle threadably received in a bore of the carburetor and rotatable to vary the location of a conical end of the needle relative to an annular seat to vary and control the area between the needle and the seat through which fuel flows. One of the major problems with needle valve assemblies is that a fuel flow change can and usually does occur after the needle valve assembly has been adjusted. This fuel flow change is caused by axial and radial movement after adjustment of the conical tip of the needle relative to the seat which may be caused by vibrations, temperature changes, installation of a limiting cap and other physical side loading of the needle. Radial movement of the needle relative to the valve seat decreases the gap between the needle and the valve seat adjacent one portion of the needle and increases the gap on the opposite portion of the needle which can drastically affect the fuel flow characteristics through the needle valve assembly.
Another problem with the needle valve assemblies is the size of the metering orifice. The annular fuel flow area between the needle and valve seat is generally on the order of about 0.001 inches to 0.002 inches wide. Most particles such as dirt or aluminum flakes within the carburetor are too large to pass through this gap and may at least partially clog the fuel flow area causing the engine to run leaner than desired.
Also, to limit the extent to which the end user can vary the fuel flow rate through a needle valve assembly, a limiter cap or the like has to be installed on the needle to limit the extent to which it can be rotated. These limiter caps increase the cost to manufacture and assemble the needle valve assemblies and may cause the needle valve to shift relative to its seat as the caps are installed after adjustment and thereby unintentionally alter the fuel flow rate of the valve assembly.
Additionally, current carburetors typically have at least a low fuel mixture needle valve, a high fuel mixture needle valve and an idle air adjustment screw. Adjustment of these components permits calibration or tuning of the carburetor to provide a fuel and air mixture to the engine at speeds ranging from idle speed to wide open throttle. However, adjustment of the carburetor is relatively complex and difficult for unskilled power equipment operators. When the user adjusts the idle or the high mixture needle, the fuel flow will change to either a richer or a leaner condition. Resulting from this mixture change, undesirable engine performance may occur such as acceleration lag, under running during deceleration, instability of the engine at idle speeds, increased exhaust emissions, and improper or less than optimum fuel to air ratios and engine performance.